Is This the First Fossil of an Embryo?

A creature called Caveasphaera lived in China 609 million years ago, and it left behind fossils that resemble tiny grains of sand. But as innocuous as those fossils appear, they may speak volumes about our own evolutionary history.

Under a microscope, the fossils turn out to be clusters of hundreds or thousands of cells. Were they on their way to developing into adult bodies? On Wednesday, a team of researchers published a study on hundreds of new Caveasphaera fossils, using high-powered X-ray beams to create three-dimensional images of the cells. They argue that these remains are embryos of early animals or their close relatives.

The scientists “certainly see things not seen before,” said Andrew Knoll of Harvard University, who was not involved in the study. “These things could be animals.”

It’s a controversial possibility, which is par for the course in studies of the origin of animals. Early animal fossils are rare and hard to distinguish from other species — made still harder when the subjects are tiny balls of cells.

“We don’t have a smoking gun,” said David Bottjer, a paleontologist at the University of Southern California who was not involved in the new study. “‘Are these animals’ is the question, and we’re still kind of saying, ‘Yeah, maybe, maybe not.’”

Animals today, from ladybugs to whales, all develop along much the same path. They begin as fertilized eggs, which multiply into a ball of cells. The cells move around within the embryo, which then grows into a multicellular body made of many different kinds of tissues.

Animals are profoundly different from their closest living non-animal relatives, which mostly live in the water as single-celled protozoans. Some cause diseases in fish; others are free-living aquatic species. Although some converge in colonies, none develop a body.

Paleontologists have looked for fossils that might shed light on how the bodies of animals evolved in such divergent directions. The search has turned up fossils belonging to many living lineages of animals, and reaching back 542 million years ago, to the Cambrian Period.

Beyond this so-called Cambrian Explosion, the search becomes a lot harder. Researchers have found a host of bizarre fronds and disks dating to around 580 million years ago. Some may belong to living lineages of animals, while others are long extinct.

But the DNA of animals offers good reason to think that the roots of the kingdom extend much further back. Over generations, genetic mutations arise at a roughly steady rate. By comparing the mutations in different animal lineages, scientists can estimate how much earlier the common ancestor of those lineages lived. The studies suggest that the common ancestor of all living animals lived somewhere around 750 million years ago — well before the Cambrian.

That’s why the possibility that a 609-million-year-old fossil like Caveasphaera is an animal is so exciting, and provokes so much controversy.

Caveasphaera was first described in 2000 by Dr. Knoll and Shuhai Xiao, then a Harvard graduate student and now a paleobiologist at Virginia Tech. The creature was among a flood of new species emerging around then from an extraordinary site in southern China called the Doushantuo Formation.

Its limestone rocks were packed with microscopic remains. A single rock from the site, dissolved in a mild acid, can yield hundreds of thousands of fossils.

Scientists including Dr. Knoll and Dr. Xiao picked out a few of the most common species to examine closely. They noticed some striking similarities between some of them and animal embryos. Some fossils were spheres containing a pair of cells, or four cells, or more.

Those first reports attracted a flood of research, and a lot of debate. It turns out that early animal embryos can be hard to distinguish from other forms of life. Red algae, for example, divide into embryo-like clusters of cells before sprouting stalks and fronds.

“I would love them to be animals,” said Philip C.J. Donoghue, a paleontologist at the University of Bristol, in England, who has studied Doushantuo fossils for 15 years. “But they could well be lots of other things, too.”

Five years ago, Dr. Donoghue and his colleagues decided to take a closer look at Caveasphaera, an organism that had been largely ignored since its discovery. They dissolved rocks and searched for fossils that had the same hallmarks as the ones described by Dr. Xiao and Dr. Knoll. The Harvard researchers had observed a distinctively small shell, inside of which they had seen a peculiar, cagelike network of cells. (“Cavea” means cage in Latin.)

Dr. Donoghue and his colleagues found hundreds of new Caveasphaera fossils and used intense beams of X-rays to take three-dimensional pictures of the cells. The researchers discovered a wide range of Caveasphaera fossils that look very much as if they are distinct stages in an embryo’s development.

In the earliest stage found by the team, a bowl-shaped cluster of cells sits at the base of the shell. The cells divided and grew around the interior of the shell until they formed the cage that gave the species its name. Then the cells moved inward, filling up the central space.

Dr. Donoghue and his colleagues argue that the new fossils make it possible to rule out some alternative explanations for Caveasphaera. Red algae, for example, do not go through this kind of development.

“It’s an important study to figure out what these fossils are,” said Dr. Xiao, who was not involved in the new research. He sees them as closely related to our own lineage. “They’re on the way to animals,” he said.

Dr. Donoghue and his colleagues see a wider range of possibilities. Recent research on the closest living relatives of animals has revealed that some can produce some cell clusters that look a lot like Caveasphaera, too.

Iñaki Ruiz-Trillo, a biologist at the Institute of Evolutionary Biology, in Barcelona, and his colleagues have studied a long-neglected group of these species, called ichthyosporeans. Some are parasites of fish, others of invertebrates. An ichthyosporean swells up, makes many copies of its DNA, then develops into 128 new cells. The cells move around inside the cluster before they break apart and swim away as single cells.

Dr. Ruiz-Trillo agreed with Dr. Donoghue that the fossils of Caveasphaera bear some striking similarities to these ichthyosporean clusters of cells. “It fits pretty well with what we see,” he said.

It’s possible, Dr. Donoghue and his colleagues believe, that Caveasphaera comes from a line of ancestors that gave rise to both ichthyosporeans and animals. If that’s the case, then the first steps toward animal embryos might have evolved long before true animals existed.

Nicholas Butterfield, a University of Cambridge paleontologist, has long been skeptical of claims that Doushantuo fossils have anything to do with animals. The new study of Caveasphaera hasn’t changed his view.

“I wouldn’t be surprised if some of these things are animals,” he said. “The question is, what can you take to the bank?” Caveasphaera could be a lot of different things, he added; he saw striking similarities between the fossils and bacteria that form clusters. “It’s so close that, if you squint, it fits quite nicely,” he said.

Dr. Donoghue is optimistic he and his colleagues will be able to zero in on Caveasphaera’s true identity. His team is finding more fossils, which he hopes will reveal more stages of its development. Filling in those gaps could reveal whether the scientists are dealing with an animal, a relative of animals, or something else entirely.

“I think the data is just out there waiting for us to find it,” he said. “It’s not a lost mastodon on a lost continent or something like that. It’s just a matter of spending a couple of years processing more material.”